The present invention relates to range or position determination. In particular, transmitters, receivers other components and/or methods of operation of a ranging or positioning system are provided.
Global navigation satellite systems (GNSS) allow a receiver to determine a position from ranging signals received from a plurality of satellites. Different GNSS systems are available or have been proposed, such as the global positioning system (GPS), Gallileo or GLONASS. The GPS has both civilian and military applications. Different ranging signals are used for the two different applications, allowing for different accuracies in position determination.
Position is determined from code and/or carrier phase information. A code division multiple access code is transmitted from each of the satellites of the global positioning system. The spread spectrum code is provided at a 1 MHz modulation rate for civilian applications and a 10 MHz modulation rate for military applications. The code provided on the L1 carrier wave for civilian use is about 300 kilometers long. The codes from different satellites are correlated with replica codes to determine ranges to different satellites. Using civilian code phase information, an accuracy of around one or two meters may be determined. Centimeter level accuracy may be determined using real-time kinematic processing of carrier phase information. A change in position of the satellites over time allows resolution of carrier phase ambiguity.
The troposphere affects the transmitted signals of a GNSS system. The troposphere may introduce errors of tens of centimeters. With meter level accuracy, such as the accuracy based on code phase measurements in the GPS system, troposphere effects may be ignored. For greater accuracy, such as associated with carrier phase ambiguity resolution in a differential system, the troposphere effects may be modeled. The models predict delays associated with propagation through the troposphere. The measured ranges are corrected by the troposphere delays, more likely increasing accuracy.
In addition to satellite-based systems, ground-based transmitters may be used for determining a range or position. Land based transmitters may include pseudolites. Pseudolite systems have been proposed for landing aircraft and determining a position of a cellular telephone. Pseudolites typically use GPS style signals or codes. Pseudolite systems which use signals similar to GNSS may be limited to several meters of accuracy if the position solution is based on uncorrected code phase measurements. Troposphere corrections may be applied to the GPS type pseudolite signals to increase accuracy. For centimeter level accuracy with pseudolites, troposphere corrections are more important.